The present invention provides novel compounds, novel compositions, methods of their use and methods of their manufacture, such compounds being pharmacologically useful in the prevention of collagen-induced or ADP-induced platelet aggregation in a mammal and in the treatment of atherosclerosis and thrombosis. More specifically, the compounds of the present invention comprise a class of ethers and thioethers of 5-methanol thiohydantoins that are unsubstituted at the No. 1 heterocyclic nitrogen atom of the thiohydantoin ring.